1. Field of the Invention
The present invention relates generally to acoustic sensors. An embodiment relates to an acoustic sensor array that has high sensitivity, a robust response, and a desired and substantially uniform buoyancy.
2. Description of Related Art
Acoustic sensors may be used to measure sound transmitted through water. One type of acoustic sensor is a hydrophone. A number of hydrophones may be coupled together to form an acoustic sensor array. Acoustic sensor arrays may be used as detection instruments to monitor vessel movement in a marine environment. Acoustic sensor arrays may be used during seismic surveys of water covered areas to estimate the location of underground formations and structures. Acoustic sensor arrays may be placed in liquid filled wellbores. Such acoustic sensor arrays may be used to conduct vertical seismic surveys. Acoustic sensor arrays may be used as sensors for a variety of other applications as well.
One type of acoustic sensor array is a liquid filled array. The acoustic sensor array, which may be over 8 kilometers in length, may include a number of active cable sections. Each active cable section may include several groups of hydrophones connected in series. The hydrophones may be placed within a flexible, sealed tubular outer jacket that is made of polyurethane or a similar material. Multiple strain members (generally between two and five) may be axially spaced apart along the length of the array within the outer jacket. The strain members may be cables; such as, but not limited to, steel cables, cables reinforced with high-strength polymers such as KEVLAR and/or cables formed of high-strength polymers such as VECTRAN. The strain members may bear the load of the array when the array is towed or otherwise supported. The array may be filled with a nonconductive, light fluid, such as kerosene, to provide the array with a desired buoyancy.
Liquid filled arrays may have several characteristics that are undesirable. The arrays may be difficult and labor intensive to construct. The arrays may have to be stored on reels that have large diameters (greater than 10 feet) to inhibit damage to the array. The arrays may have inherent sensitivity limitations due to noise generated within the array by the fluid during use. The arrays may need to be towed at depths from about 12 to 30 feet below water surface to minimize surface reflection noise and surface wave noise. The arrays may present significant safety, health and environmental problems should the outer casing leak or rupture.
A second type of acoustic sensor array is a solid or non-liquid filled acoustic sensor array. U.S. Pat. Nos. 6,108,274; 6,108,267; 5,982,708; 5,883,857; 5,774,423; 5,361,240; and 4,733,378, each of which is incorporated by reference as if fully set forth herein, describe non-liquid filled arrays and hydrophones for non-liquid filled arrays. Non-liquid filled arrays may be easier to manufacture, may be used at shallower depths and may have hydrophone responses that are more sensitive and robust than the response obtainable from a liquid filled array.
A hydrophone may produce electrical signals in response to variation of acoustic wave pressure across the hydrophone. Several hydrophones may be electrically coupled together to form an active section of an acoustic sensor array. There may be several separate active sections within an acoustic sensor array. Electrical signals from multiple hydrophones of an active section may be combined to provide an average signal response and/or to increase the signal-to-noise ratio within an active section of the array. Hydrophones may be coupled together in serial and/or parallel arrangements so that active sections of the array have a desired response and sensitivity to acoustic waves. Typically, fewer hydrophones are needed in an active section of the array if the individual hydrophones have a high signal-to-noise ratio. The use of hydrophones having high signal-to-noise ratios allows for shorter, sensitive and robust acoustic sensor arrays. Hydrophones of non-liquid filled arrays may have an increased signal-to-noise ratio as compared to typical hydrophones of liquid filled arrays.
One type of non-liquid filled acoustic sensor array is a xe2x80x9cfloatationxe2x80x9d cable design. The design includes a buoyant material, such as foamed polyethylene, that is formed over an inner jacket. The buoyant material is then covered with a polyurethane outer jacket. The types of material used for a floatation cable design may not allow the buoyant material to bind to the outer jacket or the inner jacket. If the outer casing were to be ruptured during use, water that entered into the array would undesirably be able to migrate up and down the length of the cable.
An acoustic sensor array may include a strain member, sensor sections and buoyancy sections. The sensor sections and the buoyancy sections may alternate along a length of the strain member. The sensor sections may be placed along a length of the strain member. Each sensor section may include one or more sensors capable of detecting acoustic signals. The sensors may be electronic sensors or fiber optic sensors. A potting material may be used to fill the space between a body of the sensor sections and the strain member. Buoyancy sections may be formed between adjacent sensor sections. The buoyancy sections may be formed of material that provides a desired amount of buoyancy for the array. When the buoyant sections are formed, the material of the buoyant sections may bind to the sensor sections. The material may also bind to the strain member. Filling the space between the strain members and the bodies of the sensors with potting material, and binding the buoyant sections to the sensor sections makes the array substantially an integral, solid unit. If one of the sections were to crack, migration of fluid in the array would be inhibited beyond the extent of the crack. In addition, the material used to form outer portions of the array may be made of a polymer material, such as polyurethane, that is resistant to cracking and/or breakage during use.
A sensor of a sensor section may be encapsulated within a polymer body. In an embodiment, the polymer is a polyurethane. To form a sensor section, positioners may hold a sensor at a desired position within a mold. Wiring for the sensors may be spiral wound about one, or both, of the positioners. Spiral winding the wiring may inhibit strain damage to the sensors due to bending of the array during use or during storage on a reel. The positioners may be made of, or coated with, a material that does not bind to the polymer so that the positioners may be removed after encapsulation of the sensor. The polymer is injected into the mold to form the sensor section. The mold may be part of a reaction injection molding machine. The polymer may be a material that is capable of binding to the material used to form buoyancy sections of a sensor array. The sensor sections may be formed so that the ends of the sensor sections have large surface areas that will bind to ends of buoyancy sections. A binding film may be wrapped on the ends of the sensor sections, and/or a binding fluid may be placed on the ends of the sensor sections, to enhance binding between the sensor sections and the buoyancy sections.
A sensor of a sensor section may be a hydrophone. The hydrophone may include a base that forms a back plane for a sensor. The base may have a number of ridges that form a plurality of concave surfaces in an outer surface of the base. In an embodiment, the base may have eight ridges so that the back plane has a generally octagonal cross sectional shape with eight concavely curved sides. Other back planes may have cross sectional shapes having fewer or more than 8 sides. A back plane may be molded or placed on the base, or the back plane may be an integral part of the base. A flexible diaphragm may slide over the back plane. In an embodiment, the diaphragm may have a cross sectional shape that substantially matches the cross sectional shape of the back plane yet allows air gaps to form between the concave sides and the diaphragm. In other embodiments, the diaphragm may have a different cross sectional shape than the back plane. For example, in an embodiment, the back plane has a generally octagonal outer cross sectional shape with concave curved sides and the diaphragm has a circular cross sectional shape. A sealant may be placed between the diaphragm and the back plane at both ends of the diaphragm. During use, the diaphragm is able to flex into the concavities of the back plane in response to acoustic waves that pass through the hydrophone.
A thin piezoelectric film may be wrapped around the diaphragm. The piezoelectric film may be made of polyvinylidiene fluoride. The film may include a conductive pattern. In an embodiment, the pattern is formed of a conductive ink. In other embodiments, the pattern may be formed by other techniques, such as etching. In certain embodiments, the pattern may include a number of conductive sections that are separated by voids. The voids may be positioned above the ridges of the back plane when the film is wrapped around the diaphragm and glued, taped, or otherwise sealed to the diaphragm during assembly. Small conductive traces may connect conductive sections across the voids. When an acoustic wave passes through the sensor, the wave deflects the piezoelectric film, and the film generates an electrical signal that is transmitted through the conductive pattern. Limited travel distance between the diaphragm and a concave surface of a back plane may inhibit stretching of the piezoelectric film beyond a yield limit of the film. Having only small conductive traces located at the ridges may reduce passive capacitance contributions from ridge supported portions of the piezoelectric film. A dielectric film having an outer metallic coating may be placed around the piezoelectric film as an electromagnetic shield.
The encapsulated sensors may be formed and individually tested for operational performance before being joined together in an acoustic sensor array. A strain member may be threaded through a passage within each of the encapsulated sensors. The sensors may be spaced at desired positions along a length of the strain member. Wiring of the sensors may be spiral wound around the strain member. In an embodiment, the wiring is spiral wound around the strain member in the same orientation as the winding within the encapsulated sensors. Active sections may be formed by coupling several sensors together in series and/or parallel configurations. Signal amplifiers may be coupled to the wiring where needed. The wiring of the sensors may be electrically coupled to channels in or on the strain member. In an embodiment, the sensors of an active section are connected together and there is only a single entry into a channel within the strain member regardless of the number of sensors that make up the active section. In an embodiment, the strain member includes 24 separate channels. Each active section may be tested to ensure that the active sections operate within desired parameters. For each sensor, an end of the sensor may be plugged and the space between the encapsulated sensor and the strain member may be filled with a potting material. The potting material may include filler material, such as hollow glass beads. The filler material may help establish desired buoyancy within the sensor section. The potting material may be configured to bind to a coating of the strain member and/or to the polymer material of the body of the sensor section.
Buoyancy sections may be formed between adjacent sensor sections. The sensor sections may be placed at each end of a mold. The mold may be closed and polymer may be shot or produced within the mold to form a buoyancy section between the sensor sections. In an embodiment, the mold is part of a reaction injection molding machine. In an embodiment, the mold produces buoyancy sections that have substantially the same outer circumference or perimeter that the sensor sections have. In other embodiments, the mold produces buoyancy sections that have larger or smaller outer circumferences or perimeters than the sensor sections. The larger or smaller outer circumferences may taper to the same circumference or perimeter as the sensor sections. The material used to form the buoyancy sections, which may be a polyurethane, binds to the ends of the sensors. A film or a coating may be placed on the ends of the sensor sections and/or the buoyancy sections to promote binding of the sensor sections to the buoyancy sections. The strain member may include a coating, such as a polyurethane coating, that binds with the material of the buoyancy section when the buoyancy section is formed.
The material used to form the buoyancy sections may include filler that allows the material to have a desired buoyancy. In an embodiment, the filler is hollow glass beads. The size of the hollow glass beads and the concentration of the glass beads may be adjusted so that the array has a desired overall density. Buoyancy variations in buoyancy sections of a sensor array may be desired along a length of the array to counter the effect of sections of the array that are more dense than other sections. For example, ends of an array and sections that include telemetry units may be more dense than other sections of the array. The amount and/or size of hollow beads in the material used to form the buoyancy sections adjacent to the ends and adjacent to telemetry units may be adjusted to accommodate the greater density of these sections so that the array has a desired overall buoyancy. The material used to form the buoyancy sections may be adjusted so that the array will have a substantially uniform buoyancy along a length of the array.
An advantage of a sensor array made of sensor sections, buoyancy sections and a strain member is that the materials used to form the array may have desirable properties and characteristics. For example, the primary material used to form an outermost layer of the array may be a polyurethane material that provides high resistance to physical damage to the array while still allowing the array to be flexible. The material used to form the buoyancy sections may include filler material that allows the buoyancy of the array to be controlled. The material used to encapsulate sensors within sensor sections of the array may not include filler material if the filler material will decrease an energy of acoustic waves passing through the material to the sensors.
An advantage of a sensor array made of sensor sections, buoyancy sections and a strain member is that the array may have a small outer diameter and a short length as compared to typical liquid filled arrays. Liquid filled arrays typically had to be wound on reels having diameters that were greater than 10 feet. An array formed of buoyancy and sensor sections may have a small diameter and may be stored on reels having small diameters. For example, an array may be formed that has an outer diameter of about 1xc2xd inches. Such an array may be stored on a reel having a diameter as small as about 1xc2xd feet. Sensors within solid arrays may be more sensitive than sensors within liquid filled arrays. The increased sensitivity may allow for more accurate detection of acoustic signals, and may allow for the use of arrays that are shorter than would be practical if a liquid filled array were used. The use of shorter arrays having small outer diameters allows the arrays to be stored in less space and allows the arrays to be lighter and easier to manipulate.
An advantage of a sensor array made of sensor sections, buoyancy sections and a strain member is that the sensor sections and the buoyancy sections may bind together during formation of the array. Passages through the sensor sections may be filled with potting material to seal the sensor sections to the strain member. Binding the sensor sections to the buoyancy sections and sealing the sensor sections to the strain member may inhibit fluid migration within the array should the array become cracked or damaged.
An advantage of a sensor array is that signals generated by sensors may be conveyed through electrical channels within a strain member of the array. Several sensors may be electrically coupled together to form an active section of an array. In an embodiment, each individual sensor may be coupled to a channel within the strain member. In an alternate embodiment, a group of sensors may be electrically coupled together and the group may be coupled to a channel of the sensor. Several groups may be coupled to one channel, or only one group may be coupled to one channel. Coupling only one group to a channel of the sensor array allows for a minimal number of entries into the strain member to accommodate all of the sensors of the sensor array.
An advantage of a sensor array made of sensor sections, buoyancy sections and a strain member is that controlling the material used to form the buoyancy sections may control the buoyancy of the array. In certain embodiments the material used to form the buoyancy sections may include filler. The filler may be a material, such as hollow glass beads, having a selected size and concentration that will result in the formation of buoyancy sections having a desired buoyancy. The filler material may be altered in various parts of the array to produce buoyancy sections that will accommodate different densities of the array at different locations along a length of the array. Space between the strain member and the sensor sections may be filled with a potting material during formation to couple the sensor sections to the strain member. The potting material may include filler material, such as hollow glass beads, to increase the buoyancy of the sensor sections of the array. Further advantages may include that the sensor array is strong, sturdy, durable, lightweight, flexible, simple, efficient, safe, reliable and inexpensive; yet the sensor array may also be easy to manufacture, handle and use.